In a hot dip galvanizing line, which is a line for plating a steel sheet with zinc, the steel sheet is conveyed through a molten zinc bath and then heated, so that an alloying layer of zinc and iron is formed on an outer layer of the steel sheet. At this time, it is important in terms of quality management to perform operation so that zinc plating can be alloyed in a predetermined state. That is, both an unalloyed layer in which alloying is insufficient and an overalloyed layer in which alloying is performed too much degrade quality.
Accordingly, for example, the following Patent Document 1 discloses a method to measure radiant energy by se of radiation thermometers at a plurality of points in the height direction of an alloying furnace, to specify an alloyed position by use of the obtained results of measurement of radiant energy, and to control a furnace temperature in the alloying furnace for performing alloying.
Further, the following Patent Document 2 discloses a method to install three or more radiation thermometers in an alloying furnace, and to determine an alloyed position by focusing on differences between adjacent indicated temperatures.
Furthermore, the following Patent Document 3 discloses a method to measure steel sheet radiant temperatures at a plurality of positions in a sheet temperature holding zone in an alloying furnace, and to determine an alloyed position based on emissivity calculated by use of the measurement results.